MOFs or “metal organic frameworks” are compounds having a lattice structure having vertices or “cornerstones” which are metal-based inorganic groups, for example metal oxides, linked together by organic linkers. The linkers are usually at least bidentate ligands which coordinate to the metal-based inorganic groups via functional groups such as carboxylate and/or amine. The porous nature of MOFs renders them promising materials for many applications such as gas storage and catalyst materials.
Perhaps the best known MOF is MOF-5 in which each Zn4O cornerstone is coordinated by six bis-carboxylate organic linkers. Other MOFs in which the inorganic cornerstone is for example chromium, copper, vanadium, cadmium or iron are also known. The processes of the present invention are directed specifically to zirconium-based MOFs (Zr-MOFs).
Numerous processes are known in the prior art for the production of MOFs. The most commonly used techniques involve the reaction of a metal salt with the desired organic linker in a suitable solvent, usually organic, such as dimethylformamide (DMF). High pressures and temperatures are commonly required to facilitate the reaction. Typical methods are disclosed in, for example, WO 2009/133366, WO 2007/023134, WO2007/090809 and WO 2007/118841.
The use of elevated temperatures and pressures not only increases the cost of the process but also means that scale-up to an industrial level poses many challenges. Apparatus suitable for withstanding the severe reaction conditions is often only compatible with small scale batch synthesis, rather than the continuous processes favoured for large scale production. Employing high pressures also carries with it safety concerns, particularly when combined with the use of corrosive liquids. Moreover, the use of organic solvents as the reaction medium is undesirable as such solvents are harmful to the environment and are expensive.
As MOFs become increasingly employed as alternatives to, for example, zeolites, there is a need for the development of novel processes for their production which are applicable to use on an industrial scale. Replacement of the organic solvent with an aqueous medium is reported in, for example, U.S. Pat. Nos. 7,411,081 and 8,524,932. These processes routinely involve the use of a base or require an alkaline reaction medium.
Moreover, reactions conditions which are suitable for the production of certain MOFs may not be transferable to others where different metals are used. For example, conditions found to be optimal for the production of MOFs containing main group metals from the second or third groups of the periodic table (e.g. magnesium or aluminium) are often not appropriate for the preparation of analogous frameworks wherein a transition metal is used.
The present invention is directed specifically towards Zr-MOFs. An aqueous-based process for producing a Zr-MOF is reported by Yang et al in Angew. Chem. Int. Ed. 2013, 52, 10316-10320. This process involves a two-step synthesis. The product is obtained as a gel which, to be isolated, must be washed and recrystallised. This adds to the costs and timescale of the process, making it unsuitable for use on a larger industrial scale.
There thus remains the need for the development of novel processes for the production of Zr-MOFs which are suitable for use on an industrial scale. The process should ideally be one which is “green” and thus considered environmentally friendly. It would also be advantageous to have a process which can be carried out quickly and cheaply and which offers improvements in terms of complexity over those processes already known in the art. In particular, a process which involves fewer steps is desired. A process which avoids potential corrosion problems, such as the production of hydrochloric acid (HCl) as a by-product is particularly attractive. A process susceptible to use in continuous production methods would also be advantageous. Ideally, a process which offers improvement in more than one of the above aspects would be developed.
The present inventors have surprisingly found that Zr-MOFs may be prepared in a straightforward process utilising an aqueous solvent which avoids the need for high temperatures and pressures. In particular, the specific combination of zirconium ions with sulfate ions in the reaction mixture unexpectedly leads to a procedure which is applicable to use on an industrial scale and offers an environmentally friendly and cheap route to these valuable materials.